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Guericke's Magdeburg experiment. Engraving showing the famous experiment of 1654 undertaken by Otto von Guericke (1602-86) demonstrating the strength of a vacuum. Guericke used two copper hemispheres some 3.6 metres in diameter, which were then placed together and the air inside evacuated. Guericke demonstrated the power of the vacuum (more properly of the atmospheric pressure acting on the outside) to be stronger than sixteen horses harnessed to pull the two hemispheres apart. He then opened an air tap fitted to one of the hemispheres, restoring air to the inside of the apparatus which instantly fell apart.